Copper(II) Complex of a Urea-functionalized Pyridyl Ligand: Synthesis, Crystal Structure, and Acetate Binding Properties

Article abstract of DOI:10.1002/zaac.202000175

A copper(II) acetate complex with a urea-functionalized pyridyl ligand, [CuL(OAc)₂]₂·2AcOH (1) [L = N-(3-chlorophenyl)-N'-(3-pyridyl) urea], was synthesized by the reaction of L with Cu(OAc)₂ in methanol. A zigzag-shaped hydrogen bond chain of L is obtained via urea N–H···N_pyridyl interactions, and a two-dimensional hydrogen bond network structure is further formed through the C–H···O interaction. In the complex 1, a paddle-wheel structure is generated by Cu···O_acetate interactions and Cu···N_pyridyl interactions. Furthermore, hydrogen bonding chain structure is extended through weak C–H···O hydrogen bond interactions. Through ultraviolet-visible (UV/Vis) spectroscopy, the acetate binding properties of L in solution were also evaluated. Variable temperature magnetic susceptibility measurement indicates that the metal complex 1 displays antiferromagnetic coupling property.

Full text of DOI:10.1002/zaac.202000175

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.202000175
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Copper(II) Complex of a Urea-functionalized Pyridyl Ligand:
Synthesis, Crystal Structure, and Acetate Binding Properties
Zaiwen Yang,*[a,b] Chun Li,[a] Xiangrong Liu,[a,b] Shunsheng Zhao,[a,b] Mengnan Qu,[a] Shasha Sun,[a]
Zhen Zhang,[a] Zheng Yang,[a,b] and Xinjuan Chen[b]
Abstract. A copper(II) acetate complex with a urea-functionalized
pyridyl ligand, [CuL(OAc)2]2·2AcOH (1) [L = N-(3-chlorophenyl)-NЈ-
(3-pyridyl) urea], was synthesized by the reaction of L with Cu(OAc)2
Cu···Oacetate interactions and Cu···Npyridyl interactions. Furthermore,
hydrogen bonding chain structure is extended through weak C–H···O
hydrogen bond interactions. Through ultraviolet-visible (UV/Vis)
in methanol. A zigzag-shaped hydrogen bond chain of L is obtained spectroscopy, the acetate binding properties of L in solution were also
via urea N–H···Npyridyl interactions, and a two-dimensional hydrogen evaluated. Variable temperature magnetic susceptibility measurement
bond network structure is further formed through the C–H···O interac-
tion. In the complex 1, a paddle-wheel structure is generated by
indicates that the metal complex 1 displays antiferromagnetic coupling
property.
Despite the great significant progress of anion binding prop-
erties and supramolecular structures for such ligands,[38–43] the
efficient and selective recognition of anions by synthetic prom-
ising receptors still remains a challenging subject of anion
chemistry. For this purpose, we designed a new ligand, N-(3-
chlorophenyl)-NЈ-(3-pyridyl) urea (L) (Scheme 1). Herein we
report the synthesis, characterization and the supramolecular
structures of the ligand L and its CuII acetate complex
[CuL(OAc)2]2·2AcOH (1), as well as the investigation of acet-
ate recognition of the ligand L including the binding ratio and
the association constants (Ka) by the method of the UV/Vis
spectra titrations.
Introduction
The binding, recognition, and self-assembly of anions has
become a fascinating aspect of supramolecular chemistry due
to the importance of the anions such as oxyanions and halide
anions in physiology, biology and environment.[1–5] Thus, in
recent years, the design and synthesis of simple receptors that
can bind anions specifically is the forceful chemical research
subject.[6–8] As one of the most simple and classical hydrogen-
bond donors, highly polarized N–H fragments are usually con-
tained at the binding site in the neutral synthetic receptors for
anions,[9–11] such as ureas,[12–15] thioureas,[16–19] amides,[20]
pyrroles,[21,22] and benzimidazole.[23] In these receptors, direc-
tional hydrogen bonding interactions make the receptors have
the ability to differentiate between anions with different shapes
and hydrogen-bonding requirements, which is contrary to the
only electrostatic interactions.[24–28]
The urea groups, possessing two close polarized N–H frag-
ments, have been focused as anion binding sites, owing to their
hydrogen-bonding ability that could form quite stable com-
plexes bonded with biologically significant anions such as
acetate and halide anion.[29–32] By introducing pyridyl group,
the urea-based pyridyl ligands have become a class of good
artificial receptors for ion recognition, in which the urea group
can act as hydrogen-bond donor for anions, while the pyridyl
moiety can coordinate with metal ions.[33–35] A series of such
ligands have been synthesized and extensively used to recog-
nize ions through cooperative binding of transition metals and
anions.[36,37]
Scheme 1. Synthesis of the ligand L.
Results and Discussion
Synthesis and Formulation
Synthesis and Formulation of Ligand L
The pyridylurea ligand, N-(3-chlorophenyl)-NЈ-(3-pyridyl)
urea (L), was synthesized by the reaction of 3-aminopyridine
with 3-chlorophenyl isocyanate (Scheme 1). Characterization
of L by nuclear magnetic resonance spectroscopy (1H NMR)
and infrared spectroscopy (IR) proved the synthesis of high-
* Dr. Z. Yang
E-Mail: yzwxk@foxmail.com
[a] College of Chemistry and Chemical Engineering
XiЈan University of Science and Technology
XiЈan 710054, Shaanxi, P. R. China
[b] Key Laboratory of Coal Resources Exploration and Comprehen-
sive Utilization
1
purity ligand. The H NMR spectrum of the ligand indicates
that the peaks at δ = 9.02 ppm and 8.93 ppm are from two
urea hydrogen atoms and 8.61 ppm, 8.21 ppm, 7.94 ppm, and
7.32 ppm are from four groups of hydrogen atoms on the pyr-
idine ring. Besides that, 7.71 ppm, 7.32 ppm, 7.32 ppm, and
Ministry of Natural Resources
XiЈan 710021, Shaanxi, P. R. China
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7.03 ppm belong to the four groups of hydrogen atoms on the angle 17.12°, Figure 1a). The chlorophenyl ring is
benzene ring, respectively. In addition, in the infrared spectrum almost coplanar with the urea carbonyl (torsion angle
of the ligand, the stretching vibration of N–H bonds at C6urea–N3–C7–C12chlorophenyl 6.4(4)°]. The C–N bonds of the
3343 cm–1 and 3290 cm–1 and the stretching vibration of carb- urea group are 1.362(3) Å and 1.382(3) Å, respectively. The
onyl C=O at 1717 cm–1 also provide direct evidence for the C=O bond length of urea is 1.213(3) Å, which falls within the
synthesis of the ligand.
previously reported C=O bond length of urea (1.20–
1.235Å),[44,45] indicating the formation of double bonds. In the
extended structure of L, pyridine N is linked to the urea group
of a adjacent ligand L by two bifurcated chelating N–H···N
hydrogen bonds [N2···N1i, 3.092(3) Å, N2–H2···N1i, 156.0°;
N3···N1i, 3.062(3) Å, N3–H3···N1i, 163.3°, Figure 1b and
Table 2]. Through these N–H···N bonding, a zigzag hydrogen
bond chain is further formed. The hydrogen bond chain of this
N–H···N is different from that of the self-assembly of urea-
Synthesis and Formulation of Complex 1
The metal complex [CuL(OAc)2]2·2AcOH (1) was obtained
by crystallization of the reaction product of L with 1 equiva-
lent of Cu(OAc)2. The formation of 1 can be confirmed by the
stretching vibration of N–H bonds at 3360 cm–1 and
3020 cm–1 in the IR spectrum of 1, especially the stretching
vibration of acetate at 1620 cm–1.
Crystal Structure
Crystal Structure of Ligand L
The ligand L crystallizes in the orthorhombic space group
Pca21, and the detailed crystallogrophic data are shown in
Table 1. The space group Pca21 is a non-centrosymmetric po-
lar space group, which is not often encountered. Obviously,
this is caused in this case by the orientation of the C=O bond
and C–Cl bond of the ligand. Based on the relative orientation
of the C=O bond of the urea carbonyl group and the C–Cl
bond of the aryl group, the ligand adopts a syn conformation.
One N atom of the urea group is connected by the pyridine
six-membered ring and chlorophenyl ring, respectively. The
structure of L shows that the six-membered ring of pyridine is
slightly distorted from the ring of Cl phenyl [dihedral
Table 1. Crystallographic data and refinement details for compounds
L and 1.
L
1
Empirical formula
Fw
C12H10ClN3O
247.68
C36H40Cl2Cu2N6O14
978.72
Temperature /K
Crystal system
Space group
a /Å
b /Å
c /Å
296(2)
orthorhombic
Pca21
13.578(3)
9.2763(18)
9.2307(18)
90
90
90
1162.6(4)
4
1.415
296(2)
triclinic
¯
P1
8.0883(18)
8.691(2)
15.474(3)
81.979(4)
84.152(4)
88.326(4)
1071.4(4)
1
α /°
β /°
γ /°
V /Å3
Z
Dcalc /g·cm–3
F(000)
1.517
502
1.188
512
0.314
μ /mm–1
θ range
Reflections collected
R (int)
2.195–25.085
5448
0.0206
2.367–25.046
5397
0.0264
R1; wR2 [I Ͼ 2σ(I)]
R1; wR2 (all data)
GOF (F2)
Flack-parameter
0.0273; 0.0694
0.0300; 0.0713
1.052
0.0518; 0.1384
0.0722; 0.1652
1.060
Figure 1. Crystal structure of the ligand L. (a) ORTEP drawing; (b)
Chain obtained by N–H···N hydrogen bonds. (c) 2D hydrogen bond
sheet network linked by C–H···O hydrogen bonds. Symmetry codes:
0.02(3)
/
i
ii
1–x, 2–y, z + 0.5; x–0.5, 2–y, z.
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urea formed by N–H···O hydrogen bond, which is common in 2.6432(11) Å, which falls into the same order of magnitude
urea-based ligands.[46,47] It is indicated that the self-assembly as described in the literature (2.612–2.713 Å).[50–54] Adjacent
of pyridyl-urea by N–H···N hydrogen bond (in the presence complex 1 units are held together by the C–H···O hydrogen
of pyridylurea) is more stable than urea-urea self-assembly. bonds [C4···O1iii, 3.362(6) Å, C4–H4···O1iii, 144.5°, Figure 2c
Furthermore, the 1D hydrogen bonds structure further self-as- and Table 2], forming a hydrogen bond chain structure includ-
sembled via C–H···O interactions [C1···O1ii, 3.314(3) Å, C1– ing paddle-wheel units. Such Cu2(COO)4 paddle-wheel clus-
H1···O1ii, 140.7°] involving carbonyl oxygen atom of urea ters have been used as secondary building units (SBUs)
functionality (Figure 1c and Table 2), leading to an infinite 2D for the construction of porous metal–organic frameworks
hydrogen bond sheet network structure.
(MOFs).[55–57]
Table 2. Selected hydrogen bond parameters /Å,° for L and 1.
D–H···A
H···A
D···A
D–H···A
L
1
N2–H2···N1i
N3–H3···N1i
C1–H1···O1ii
N2–H2···O7ii
N3–H3···O7ii
C4–H4···O1iii
2.29
2.23
2.54
3.092(3)
3.062(3)
3.314(3)
156.0
163.3
140.7
2.16
2.07
2.56
2.953(6)
2.884(6)
3.362(6)
153.6
157.3
144.5
i
ii
Symmetry codes for L: 1–x, 2–y, z + 0.5; x–0.5, 2–y, z. Symmetry
ii
iii
codes for 1: x–1, y, z; 1–x, –y, –z.
Crystal Structure of Complex 1
X-ray single crystal diffraction results of complex 1 indicate
¯
that 1 crystallizes in the triclinic space group P1 and the de-
tailed crystallographic data are exhibited in Table 1. In com-
plex 1, the asymmetric unit comprises two acetate ions and
one L ligand coordinated to the Cu2+ ion, and one CH3COOH
molecule (Figure 2a). Compared with ligand L, the bond
length of C=O in urea of complex 1 is 1.215(6) Å, which is
slightly longer than that of ligand L, and the selected bond
lengths and angles for 1 are listed in Table 3. The CuII ion is
coordinated by two carbonyl oxygen atoms from acetate
groups and one pyridine N atom from L [Cu1···O2, 1.970(3)
Å, Cu1···O4, 1.968(3) Å, Cu1···N1, 2.193(4) Å, Figure 2a and
Table 3]. The Cu–O bond length values are slightly shorter
than that (1.980(6) Å] found in [Cu2(phend)2(μ-OAc)4].[48]
Among them, one carbonyl oxygen atom is on the same plane
as L, and the other is distorted [O2–Cu1–O4, 88.61(15)°, Fig-
ure 2a and Table 3]. The other O atom of the carboxyl group
is linked to the adjacent Cu atom to form a unit (monomer)
with two ligands L molecule, two Cu(OAc)2 molecules and
two CH3COOH molecules (Figure 2b). The urea groups of the
two ligands act as hydrogen-bond donors to form two pairs of
bifurcated chelating N–H···O hydrogen bonds with the carb-
onyl oxygen atoms of CH3COOH, respectively [N2···O7ii,
2.953(6) Å, N2–H2···O7ii, 153.6°; and N3···O7ii, 2.884(6) Å,
N3–H3···O7ii, 157.3°, Figure 2b and Table 2]. The pyridyl N
donors of two ligands are coordinated to a pair of CuII ions in
the axial positions to generate the paddle-wheel units, while
two acetate groups from two different Cu(OAc)2 groups are
located in the equatorial plane [Cu1···N1, 2.193(4) Å,
Cu1···O2, 1.970(3) Å, Figure 2b and Table 3]. The copper
atom (Cu1) is slightly out of the square plane of oxygen atoms
(0.207 Å), which is consistent with the theory that the longer
the Cu···Cu distance within the dimer, the higher the Cu out-
Figure 2. Crystal structure of the complex 1. (a) ORTEP drawing of
asymmetric unit of 1. (b) Monomer of 1 with a paddle-wheel structure.
(c) Chain structure held together by the C–H···O hydrogen bonds in-
i
cluding paddle-wheel units in 1. Symmetry code for 1: –x, –y, 1–z;
ii
iii
x–1, y, z; 1–x, –y, –z.
Table 3. Selected bond lengths /Å and angles /° for L and 1.
Bond
Dist.
Bond
Angle
L
1
C6–O1
C6–N2
C6–N3
1.213(3)
1.382(3)
1.362(3)
O1–C6–N2
O1–C6–N3
N2–C6–N3
123.7(2)
125.5(2)
110.8(2)
Cu1–O2
Cu1–O3i
Cu1–O4
Cu1–O5i
Cu1–N1
Cu1–Cu1i
C6– O1
C6–N2
1.970(3)
1.969(3)
1.968(3)
1.963(3)
2.193(4)
2.6432(11)
1.215(6)
1.370(6)
1.362(6)
O2–Cu1–N1
O3– Cu1–N1i
O4–Cu1–N1
O5–Cu1–N1i
O2–Cu1–O4
O2–Cu1–O3i
O1–C6–N2
O1–C6–N3
N2–C6–N3
96.92(13)
95.13(13)
95.37(13)
96.81(13)
88.61(15)
167.92(13)
122.9(4)
124.0(4)
113.2(4)
C6–N3
of-plane deviation.[49] In complex 1, the Cu1···Cu1i distance is Symmetry code for 1: –x, –y, 1–z.
i
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Acetate Binding Properties of L
Binding Constant K of the Ligand L to AcO– Anion
Binding Mode of the Ligand L to AcO– Anion
The binding constant of L with AcO– anion was analyzed
in CH3CN by UV/Vis titration at room temperature. The solu-
tion of AcO– in CH3CN solution of ligands (0.01 mM) was
prepared for the UV/Vis titration (Figure 4a). Small portions
(3 μL) of the AcO– solutions were added to the solution of
L (3 mL, 0.01 mM). UV/Vis titration spectra reveal that upon
addition of AcO– to L, the high absorption peak at 256 nm
undergoes a red shift to 266 nm. An isosbestic point at 260 nm
suggests that the appearance of the stable complex. With the
addition of AcO–, the absorbance at 266 nm increases grad-
ually. The binding constant K of L with AcO– was fitted by
Origin nonlinear least-squares through the following for-
The binding mode of L with AcO– anion was studied in
CH3CN by UV/Vis spectra. Keeping the total concentration of
L with AcO– is 1ϫ10–5 mol·L–1, and the molar ratio of host
to guest was changed in the following turn : 10:0, 9:1, 8:2,
7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, 0:10. All the UV/Vis spectra
of molar ratio of host to guest were acquired (Figure 3a), and
Job’s plot[58–60] of molar fraction xL of L vs. the concentration
of the corresponding complex ([HG]) were obtained. The ab-
sorbances at the wavelength of 266 nm were used to calculate
the complex concentration, [HG] = (A0 – A) / A0 ϫ[H], where
A0 stood for the absorbance of the free ligand, A was the ab-
sorbance of the solution after adding AcO– anion, [H] repre-
sented the total concentration of the corresponding ligand L.
Job’s plot analysis revealed a 1:1 complexation mode between
the ligand L and AcO– anion (Figure 3b).
mula:[61–63] X = X0 + (Xlim – X0)/2cH{cH + cG + 1/K – [(cH
+
cG + 1/K)2 – 4cHcG]1/2}, where the X0 represented the ab-
sorbance of L at 266 nm, X and Xlim stood for the absorbance
of L at 266 nm in the presence of an anion and its limit value,
Figure 3. (a) UV/Vis spectra of L with AcO– in CH3CN, H = L, G = AcO– (in the form of tetrabutylammonium acetate (TBAOAc), [10H] =
[10G] = 1ϫ10–5 mol·L–1. (b) Job’s plot analysis for the L with AcO– in CH3CN at the wavelength of 266 nm in CH3CN.
Figure 4. (a) UV/Vis spectra of L before and after titration with various amount of AcO– (0–10 equiv.) in CH3CN. (b) The fitting curve of the
titration at 266 nm in CH3CN.
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Figure 5. Thermal dependence of the magnetic susceptibility (xM) and (xMT) vs. T for the metal complex 1 at 0.1 T applied field in the
temperature range 2–400 K.
respectively, K is the binding constant of L with AcO–, cH is reaction of N-(3-chlorophenyl)-NЈ-(3-pyridyl) urea (L) with
the initial concentration of L and cG is the concentration of Cu(OAc)2. Unlike the two-dimensional infinite hydrogen bond
AcO– during titration. The fitting curve reveals that the binding network formed by the interaction of the ligand L through
constant K of L with AcO– is K = 1.505ϫ104 m–1 (R2
=
Nurea–H···Npyridyl interactions and C–H···O interactions, com-
0.9906) (Figure 4b).
plex 1 forms a special paddle-wheel-like structure through the
The stoichiometric 1:1 complex formation between receptor interaction of Cu···Npyridyl interactions and Cu···Oacetate interac-
L and AcO– anion is consistent with that of receptors con- tions, which further forms an infinite hydrogen bonding chain
sisting of urea binding site.[31,43] Such hydrogen bonding inter- extended by C–H···Ourea hydrogen bonds. In addition, the bind-
actions between urea groups and acetate anions have been used ing ratio of the ligand to acetate anion 1:1 and the binding
to make the ion-selective electrode for acetate based on a por- constant of K = 1.505ϫ104 m–1 are obtained by Job’s plot
phyrin ionophore,[64] to operate the translational isomerism of experiments and UV/Vis spectroscopy titration. Variable tem-
the interlocked molecular switch reversibly through the se- perature magnetic measurement of complex 1 exhibits an anti-
quential addition and removal of acetate anions,[65] and to con- ferromagnetic interaction between the carboxylato-bridged di-
struct the colorimetric chemosensor for naked-eye detection of nuclear CuII ions. The investigation of structure and related
acetate anion.[66,32] The study of acetate anion binding proper- properties of the acetate-bridged dinuclear CuII complex pro-
ties may facilitate to understand the mechanism of acetate cell- vides useful information for its applications in anion chemistry
metabolism in physiology.[67–69]
and physiology.
Experimental Section
Magnetic Measurement of 1
Materials and Instruments: All solvents for the syntheses (analytical
grade) were used without further purification, and the metal salt
Cu(OAc)2, 3-chloro-phenylisocyanate, and 3-pyridyl-amine were com-
mercially available. 1H NMR spectra were recorded on a Bruker
400 MHz spectrometer using tetramethylsilane (TMS) as an internal
standard. IR spectra were measured with an IR spectrometer equipped
with a Smart Omni-Transmission accessory (Thermo Scientific). The
UV/Vis spectra were measured in acetonitrile solution by PERSEE
TU-1950 Ultraviolet-Visible Spectrophotometer. Magnetic measure-
ment was carried out with a physical property measurement system
(PPMS-9, Quantum Design).
Variable temperature magnetic susceptibility measurement
of the metal complex [CuL(OAc)2]2·2AcOH (1) was carried
out and the result is shown in Figure 5. Upon lowering the
temperature, the magnetic susceptibility (xM) first increases
very smoothly and then decreases rapidly, with a broad maxi-
mum around 300 K (Figure 5a). This behavior is characteristic
for antiferromagnetic coupling within the carboxylato-bridged
pairs of CuII forming the copper complex.[70] As shown in
Figure 5b, the decreasing xMT value from 0.33 to
0.01 cm3·K·mol–1 upon lowering the temperature from 300 to
60 K also indicates the existence of antiferromagnetic coupling
when the spins become more and more constrained.[71] This
behavior is consistent with the syn-syn, equatorial-equatorial
arrangement of the carboxylato-bridges around copper cen-
ters.[72]
Synthesis of the Ligand L: An acetonitrile solution (50 mL) of 3-
chloro-phenylisocyanate (1.32 g, 8.6 mmol) was dropped into an
acetonitrile solution (50 mL) of 3-pyridyl-amine (0.81 g,8.6 mmol).
The mixture was refluxed for 2 h and the precipitate thus obtained was
filtered off and washed with diethyl ether, and then dried under vac-
uum to yield L as a white solid (1.88 g, 88%). The ligand L was
dissolved into the methanol and then filtered. The clear solution was
volatilized and concentrated at room temperature for several days to
obtain single-crystals suitable for X-ray diffraction. M.p.: 184–185 °C.
IR (KBr): ν˜ = 3343 (N–H), 3290 (N–H), 3055, 1717 (C=O), 1596,
Conclusions
In summary, we have demonstrated that a copper(II) acetate
complex [CuL(OAc)2]2·2AcOH (1) can be synthesized via the 1541, 1475, 1295, 1268, 1190, 704 cm–1. 1H NMR ([D6]DMSO,
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Synthesis of the Acetate Complex [CuL(OAc)2]2·2AcOH (1): L
(70 mg, 0.28 mmol) and the metal salt Cu(OAc)2 (51.3 mg,
0.28 mmol) were added to the methanol (50 mL). After refluxing for
about 2 h, the reaction solution was cooled down to the room tempera-
ture and then filtered. The clear solution was volatilized and concen-
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suitable for X-ray diffraction. Yield: 52%. M.p.: 208–209°C. IR
(KBr): ν˜ = 3360 (N–H), 3020 (N–H), 1730 (C=O), 1620 (acetate),
1590, 1480, 1200, 702 cm–1.
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Acknowledgements
The work was supported by the National Natural Science Foundation
of China (no. 21301139, 21073139, 21103135), the China Scholarship
Council (no. 201808615050), and the Youth Innovation Team of
Shaanxi Universities, Natural Science Basic Research Plan in Shaanxi
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Received: April 21, 2020
Published Online: ᭿
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Journal of Inorganic and General Chemistry
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Z. Yang,* C. Li, X. Liu, S. Zhao, M. Qu, S. Sun, Z. Zhang,
Z. Yang, X. Chen ............................................................... 1–8
Copper(II) Complex of a Urea-functionalized Pyridyl Ligand:
Synthesis, Crystal Structure, and Acetate Binding Properties
Z. Anorg. Allg. Chem. 0000, 0–0
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